Chemical ionization mass spectrometry has been used for analyte identification since the development of mass spectrometry [Munson, M. S. B.; Fjeld, F. H. J. Amer. Chem. Soc., 88, 2621-2630 (1966); Munson, M. S. B.; Field, F. H. J. Amer. Chem. Soc., 88, 337-4345, (1966); Munson, M. S. B.; Field, F. H. J. Amer. Chem. Soc., 89, 1047-1052, (1967)]. In recent years instruments utilizing chemical ionization sources have been marketed for analysis of gas mixtures [Linforth, R.; Preece, S. Technical Note 236, Micromass Ltd, (1998); Adechy, M.; Shress, V.; Squibb, A. Peak, 1, 2-4, (2000)]. The combination of a chemical ionization source with flow tube reactor techniques allows both identification and quantification of trace species without internal calibration. Two closely related systems have been developed using this combination of techniques; Proton Transfer Mass Spectrometry [Lindinger, W.; Hansel, A.; Jordan, A. Int. J. Mass Spectrom; Jon Proc., 173, 191-241, {1998)] and Selected Ion Flow Tube Mass Spectrometry [Spanel, P.; Smith, D. Mea Bioi. Eng. Comput., 34, 409-419, (1996)].
The selection of the chemical ionizing agent or precursor ion is critical to the application of the technique. The precursor ion must be reactive with the analyte molecule(s) but unreactive with the bulk gas within which the analyte is present as a trace component. In most applications to date the bulk gas is ambient air or exhaled breath and the analyte species are various Volatile Organic Compounds (VOCs).
Precursor ions which have been in common use include NO+, O2+, NH4+ and OH− A further important criterion for selection of a chemical ionization precursor is the facility with which it can be generated in large amounts using typical instrumentation such as an electron impact or microwave discharge source.
It is known from previous experimental work (Freitas, M. A. O'Hair, R. A. J. Int. J. Mass Spectrom; Ion Proc 1998, 175, 102-122) that the methoxymethyl cation is not reactive with alkanes but will react with molecules containing heteroatoms. However, the use of CH3OCH2+ as a chemical ionization reagent for quantification and identification of analytes has not been tried previously.